1. Field of the Invention
This invention relates to an optical glass for precision molding, capable of carrying out a precision molding at a low temperature and needing no polishing and grinding after the precision molding.
2. Description of the Prior Art
Since the prior art optical glass of SF type (high referactive index, high dispersion) containing a large amount of lead oxide in the glass composition is very stable and has a relatively low softening point, the precision molding thereof is carried out at a low temperature zone. JP-A-1-308843 discloses a glass composition rendered feasible at a further low temperature as an optical glass for precision molding. JP-A-7-247135 and JP-A-7-247136 disclose glasses of P.sub.2 O.sub.5 --PbO--Nb.sub.2 O.sub.5 --WO.sub.3 -alkali metal oxide type each containing a large amount of lead oxide and JP-A-8-157231 discloses P.sub.2 O.sub.5 --B.sub.2 O.sub.3 --Nb.sub.2 O.sub.5 --WO.sub.3 -alkali metal oxide type.
However, the precision molding is generally carried out in a reducing atmosphere so as to prevent a mold from oxidation, so if lead oxide is contained in the glass composition, the lead oxide on the glass surface is reduce and deposited as lead on the surface of a precision molding lens. The lead is evaporated by heating for the precision molding, a part of which adheres to the surface of the mold material to form a convex part which is then transferred to the surface of the precision molding lens as it is. When such step. are continuously repeated, the surface accuracy of the precision molding lens cannot be maintained, so that not only such optical properties as designed can not be obtained, but also an operation for removing the lead adhered to the mold is required. This is not suitable for mass production of the lenses. Furthermore, because of containing lead oxide in large amounts, the specific gravity is increased and thus, there arises another problem that weight-lightening of an optical part is difficult in which these lenses are incorporated. Accordingly, the glasses disclosed in the foregoing JP-A-1-308843, JP-A-7-247135 and JP-A-7-247136 are not suitable, nor practically used as an optical glass for precision molding.
On the other hand, in a glass described in JP-A-8-157231, surely, lead oxide is not incorporated, but in place of the lead oxide, TiO.sub.2 is all incorporated, although claimed as an optional component, in order to obtain high refractive index, high dispersion properties as shown in Examples 7 to 11 of the second embodiment of the present invention. Consequently, the resulting glass is very strongly colored. In the ordinary optical systems, use of a single glass lens is not realistic and many optical systems are substantially composed of a number of lenses. Thus, it is desired that coloration of these glass lenses is rendered as little as possible. Accordingly, the glass described in JP-A-8-157231 is substantially caused to have high referactive index, high dispersion properties by TiO.sub.2, which cannot be said to be preferable from the standpoint of optical designing.
In the case of known mold materials for precision molding, there arises a problem that the higher is the precision molding temperature, the more is oxidation or deterioration of the mold material, thus resulting in difficulty in maintenance of the surface accuracy of the mold material and in mass production of lenses by the precision molding. On the other hand, the precision molding is generally carried out at a temperature of higher by 15 to 50.degree. C. than the yield temperature (At) of the glass. That is, an optical glass to be subjected to precision molding must be precision molded at a temperature of as lower as possible and to this end, it is desired that the yield temperature (At) measured by TMA of the glass is as lower as possible In the foregoing JP-A-8-157231, however, there are no examples concerning a glass having a high refractive index, high dispersion properties (refractive index (nd) at least 1.83, Abbe number (.nu.d) at most 26.0) and low softening property (yield temperature (At) at most 550.degree. C.). In this specification, the yield temperature is defined based on the measurement by TMA.
Moreover, the inventors have developed an invention relating to a new optical glass in JP-A-10-316448. In the optical glass of P.sub.2 O.sub.5 --Nb.sub.2 O.sub.5 --TeO.sub.2 -alkali metal oxide type, described in JP-A-10-316448, our expected objects can substantially be achieved, but in view of influences upon the ambient environment or the human body by TeO.sub.2 contained in a glass composition, it is concluded that further improvements are required.